Compressors of this type are used in particular in turbojet engines and are constituted by a rotor comprising either a succession of separate disks that are stacked one after another, or else a single drum designed to receive the series of blades constituting the various stages.
Conventionally, the rotor includes recesses or retention grooves that are made by machining in order to form spaces between pairs of adjacent stages in which the blades of the stator stages are received, the blades being secured to a stationary portion presenting a casing.
The casing forms a segment of the radially outer boundary of the filament of flow along which air flows through the turbomachine.
In conventional manner, the moving blades are secured individually to the drum via housings that are regularly distributed and equal in number to the blades, the shape of the housings being determined so as to cooperate with the roots of the blades by being complementary in shape, thereby ensuring that the blades are held radially, e.g. by a fastening of the dovetail type. To hold each blade root in translation relative to its housing, in particular in an axial direction, each blade is usually held individually by a system involving a ball, a pin, a staple, a flange, a spacer, etc.
While a turbojet engine is in operation, in particular with present-day civilian engines, and given the temperatures and pressures that are reached by the hot air, it is necessary to provide a regulation function in the event of surging.
Surging is a phenomenon within the engine that should be avoided since it involves sudden oscillations in air pressure or air flow rate, thereby subjecting the blades to considerable levels of mechanical stress that can cause them to be weakened or even broken. This phenomenon occurs in particular at the blade tips, in the boundary layer of air that is present between the blade tips and the casing, and it gives rise to local pockets of reduced pressure, leading to the so-called “cavitation” phenomenon.
At present, this function of regulating surging is provided by bleed or unloader valves which enable this boundary layer to be sucked away, thereby degrading engine rating, while also serving to evacuate water and/or ice that might have penetrated into the engine, in particular with compressors operating at high pressure. Nevertheless, such bleed or unloader valves are relatively expensive and fragile, and require an electrical power supply and strict maintenance.
Proposals have also been made to pierce the casing so as to form controlled leaks: nevertheless, that solution is very harmful in terms of energy efficiency since it puts the air stream directly into communication with the outside of the filament of flow.